Hi Ian
That experiment confirms that WHATEVER configuration of
other earths is used, the system prefers to have the coil earthed back
along
the screen of the coax and then by any tortuous route you care to use
through the shack to the mains and the station earth. Any attempt to go
direct to the special point so thoughtfully provided by a caring owner and
it goes into conniptions.
I have found RF earthing or grounding to be a bit of a black art -
particularly from a suburban location.
I have three earthing systems
Three 10ft long 2 inch diameter copper poles sunk into the ground using the
Crawley radio club special hole borer.
Wire netting covering all the lawn some distance around the antenna
feedpoint. I have a special deal with the earthworms - they helped bury the
wire netting in the spring/summer months and I keep them warm in the winter
with LF transmissions.
A mass of thick copper wire around the base of the mast. This was included
when the mast was built (long before LF) as part of the lightning protection
system.
Unlike Ian's system I find that I get increases in current as each of the
earthing systems is added to the bottom of the coil.
It wasn't always so. When I used the BK amp originally I used a tapped
loading coil arangement and got some strange effects. Things got better when
I started using a multi-tapped torroid transformer, which isolated the BK
amp from the antenna system. This transformer was located at the base of the
coil and fed at 4ohms from the amp using heavy duty electrical cable. The BK
amplifier is prone to instability when experimenting with different loads.
This is caused by the high gain of the amplifier (the negative feedback
might also become positive feedback with some loads) and could be fixed by
putting some heavy resistive loading at the input of the amplifier.
The Decca transmitter has a power combiner transformer, which also provides
some isolation. I now use a multi tapped torroid auto transformer at the
base of the coil to match the transmitter to the antenna.
I think the use of elevated radials as mentioned by Mal and Alex is a non
starter for the reasons mentioned by Ian. The other reason is space, or the
lack of it.
The exception I found was at Amberley museum where the ground is mostly
chalk but there is plenty of space. I used an elevated radial system of a
couple of long wires whose height above the ground was determined by
available trees and bushes. I tried loading the radials. Again strange
effects. The radial and antenna coils had to be adjusted so that the
floating matching and resonating controls were at a reasonable low RF
potential or you could get zapped.
Appendix 1 of the LF experimenters book illustrates the amount of effort
that went into LF Earthing systems and that underground systems were
favoured for most commercial stations for reasons stated
Regards,
Peter, G3LDO
e-mail <[email protected]>
Web <http://web.ukonline.co.uk/g3ldo>
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